Synthesis And Surface/Interface Stability Of Garnet-Type Solid-State Electrolyte

Given the increasing demands for next generation energy storage systems with high energy density,environment friendly,and long life time,Li-S、Li-air、Li-redox flow batteries applead unprecedented attentions and development.Inorganic flame-retardant solid-state electrolytes have been intensively studied as the desirable separator electrolytes due to their excellent mechanical stability,high Li-ion conductivity and wide electrochemical window.Among them,garnet-type Li⁺ions conductors have appealed considerable attention because of their outstanding compatibility with Li metal anode.To date,the garnet-type solid-state electrolytes synthesized have reached industrial standards through modifications of preparation process and intentional doping.Further research should be dedicated to stability and interface behaviors of garnet-type solid-state electrolytes for their application in lithium batteries with high energy power and long life span.In this work,Li-stuffed garnet-type solid-state electrolyte,Li₇La₃Zr₂O₁₂（LLZ）,with high crystallinity was prepared via high temperature solid-state reaction process by adjusting the sintering steps and temperatures.The effect of sintering procedure on phase composition,density and Li-ion conductivity was studied.After the starting materials are heated to900℃and kept for 6 h,LLZ of tetragonal phase is obtained;when the temperature is raised to 1100°C,the transformation from tetragonal phase to cubic phase begins and is complete after holding for 6 hours;the densification process is accomplished by keeping at 1200℃for 24 h.The as-prepared LLZ with relative density of 94%exhibits a Li-ion conductivity of 4.5×10^-4 S?cm^-1 at room temperature.After that,the conductivity is improved to 6.45×10^-4 S?cm^-1 and the electrochemical window is enlarged to 0⁶ V by Ta⁵⁺doping.Stability of solid-state garnet electrolyte in air was further studied.The results suggest that the microstructure and Li-ion conductivity vary when the Li_6.5La₃Zr_1.5Ta_0.5O₁₂（0.5Ta-LLZ）pellets are aged in air with different humidity.The lattice of 0.5Ta-LLZ expanded after 6-month exposure to humid air（humidity≈85%）.Li₂CO₃ was observed on the surface,and the Li-ion conductivity dropped from 6.45×10^-44 S?cm^-1 to 3.61×10^-44 S?cm^-1.However,no significant change was found in samples exposed to dry air（humidity≈5%）.The reaction mechanism of garnet（grain and grain boundary in 0.5Ta-LLZ）in humid air was proposed based on the above observations.Sintering aid LiF was used to improve the stability of garnet grain and grain boundary.Formation of Li₂CO₃ was suppressed and the special area resistance of LLZT-2LiF/Li was reduced to 345??cm²,which is one third of that of the LiF-free LLZT.In traditional all-solid-state lithium batteries,solid-state LLZT-2LiF electrolytes are in poor contact with Li metal.Here,a Li-ion conductive adhesive polymer was employed to improve the contact between solid-state electrolyte and Li metal,achieving a total resistance of all-solid-state lithium battery less than 1500?.The initial discharge capacity reached 142 mAh?g^-1 at a current density of 80μA?cm^–2.The coulombic effeciency was maintained at 99.8–100%after 100 cycles.By applying second-phase sintering aids Li₃BO₃,Li₄SiO₄,and Li₃PO₄,pores in garnet grain boundaries were filled.It is observed that by adding Li₃BO₃ the sintering performance of garnet is improved;by adding Li₄SiO₄ the compactness is enhanced;while adding Li₃PO₄ is helpful for refining garnet grains and thus reducing interfacial resistance with Li metal.The Li/LLZT-LPO/Li symmetric cells performed stable Li⁺stripping and plating at 100μA?cm^-2 for 66 h,while the Li₃PO₄-free cell short-circuited within 3 h.The effect of Li₃PO₄ in LLZT is probably resulted from a redox reaction occurring with Li₃PO₄ during charge/discharge process,yielding a Li-ion conductive phase Li₃P,which mitigates the Li⁺flux at LLZT-LPO/Li interface and thus suppresses the growth of lithium dendrites.To investigate the electrochemical properties of garnet-type electrolyte in cells,Li-LiFePO₄ hybrid cells were assembled with LLZT solid-state electrolyte and minor liquid electrolyte.The hybrid cell showed a severe capacity deterioration after 20 cycles.The interfacial impedance of dual electrolytes increased with time and the Li-ion conductivity of the liquid-electrolyte（LE）-treated LLZT decreased.Li⁺/H⁺exchange and formation of Li₂CO₃ are confirmed to be the primary factors for the performance degradation.In this work,LLZT-LE interface is stabilized by n-BuLi which is a superbase that can withdraw H⁺aggressively from surroundings.The hybrid cell with n-BuLi exhibits great charge and discharge stability,and the capacity maintains over 95%of the initial discharge capacity after cycling at both 100 and 200μA?cm^-2 for 200cycles.